1. Field of the Invention
The present invention is related generally to large single-crystal quaternary alloys of the IB-IIIA-Se.sub.2 type having chalcopyrite structure. More specifically it relates to large single crystal chalcopyrite alloys of Cu.sub.x Ag.sub.(1-x) InSe.sub.2 and CuIn.sub.y Ga.sub.(1-y) Se.sub.2, where x is in the range of about 0.45 to about 0.9 and y is in the range of about 0.8 to about 0.98. The invention also relates to methods for preparing such large, high-quality single-crystal alloys of the same having chalcopyrite structure.
2. Description of the Prior Art
Although the full extent of their potential is not presently known, crystal alloys of copper-indium-diselenide (CuInSe.sub.2) are of interest in the semiconductor industry as potential semiconductor material, especially for heterojunction type semiconductors and as candidates for photovoltaic power systems and in opto-electronic applications.
In the not too distant past, the process of producing CuInSe.sub.2 was cumbersome and inadequate. However, the inventor of the present application has in copending U.S. patent application Ser. No. 06/676,343, filed Nov. 29, 1984, provided an improved method for producing CuInSe.sub.2. This method is also disclosed by the inventor in J. Electron. Mater., Vol. 14, pp. 451 (1982). Although set forth in greater detail and with particularity in that application and article; the method in general comprises the steps of placing in a refractory container a reaction mixture of copper (Cu), indium (In), and selenium (Se) calculated by atomic percent to produce the desired composition. Additionally boric oxide (B.sub.2 O.sub.3) is placed over the Cu, In, and Se reaction mixture in a sufficient quantity so that when the B.sub.2 O.sub.3 is melted it will cover and encapsulate the Cu, In, and Se in the container. The container with the reaction mixture and B.sub.2 O.sub.3 is placed in a closed, high-pressure chamber with an inert pressurized gas environment, such as argon or helium gas. The chamber is pressurized to reduce the vaporization of Se, which is otherwise easily vaporized. The container and its contents are then heated to at least 1005.degree. C. and preferably to about 1025.degree. C. to 1100.degree. C. to melt the B.sub.2 O.sub.3 and melt and react the reaction mixture. The melt is held at such elevated temperatures under inert gas pressure for several hours to synthesize and equilibrate the reaction mixture to CuInSe.sub.2.
As taught in the referenced copending application, the crystal structure of CuInSe.sub.2 can then be grown in two ways. In one crystal growth method, where directional solidification or Bridgman/Stockbarger-type growth is desired, a seed crystal is inserted into the crucible and partially melted to initiate single-crystal growth and the temperature is then slowly lowered. If a single-crystal structure is required, the cooling rate is controlled, and should be in the range of about 5.degree. C./h to obtain proper ordering through the sphaleritic phase to the chalcopyrite phase. The cooling rate can be faster if a single-crystal structure is not required. In another method, the Czochralski crystal growth method, a seed crystal of CuInSe.sub.2 is inserted through the encapsulating B.sub.2 O.sub.3 melt to contact the CuInSe.sub.2 melt. The melt temperature is then adjusted until crystal growth begins to occur on the seed. The seed is then raised upward, with rotation, at a speed of not more than about 10 mm/h to grow the crystal.
As noted above, CuInSe.sub.2 has been investigated as a candidate material for thin-film photovoltaic power systems and for opto-electronic applications. CuInSe.sub.2 is known to have utility for such applications, and especially for photovoltaic usage as it is a direct band-gap semiconductor with an optical absorpotion coefficient greater than 10.sup.5 cm.sup.-1 over most of the solar spectrum and has an unusually steep absorption edge near 1 eV. Therefore CuInSe.sub.2 absorbs most incident light in a thickness of 1 .mu.m. Thin solar cells of CuInSe.sub.2, which operate at approximately 11% efficiency, have been made in the laboratory. However, it appears that the band-gap energy of CuInSe.sub.2, which is about 1.05 eV, is too small to allow significantly higher photovoltaic conversion efficiencies despite the excellent absorption characteristics of the material. Methods of increasing the band-gap energy of CuInSe.sub.2 by alloying have been attempted. For example, in 1979, a technique using spray pyrolysis to form and study thin films of gallium modified copper-indium diselenide alloys (CuIn.sub.y Ga.sub.(1-y) Se.sub.2) was published by B. R. Pamplin in Prog. Crystal Growth Charact. Vol. 1, pp. 395-403. This work verified that the band-gap energy of the CuIn.sub.y Ga.sub.(1-y) Se.sub.2 alloys does increase as y decreases (from 1 to 0). However, the resulting alloy films were extremely thin, several microns or less, were polycrystalline in nature, had a grain size on the order of several microns or less, and had sphalerite structure. Also in 1979 Chapman, et al. reported in Appl. Phys. Letters, Vol. 34, pp. 735-737 on the fabrication of a sintered powder specimen of silver modified copper-indium-diselenide (Cu.sub.0.5 Ag.sub.0.5 InSe.sub.2), and used the specimen for property measurements. However, the resulting sintered powder was neither an alloy nor a crystal. Presently no method is known or available for obtaining large single-crystal alloys of Cu.sub.x Ag.sub.(1-x) InSe.sub.2 and CuIn.sub.y Ga.sub.(1-y) Se.sub.2 having chalcopyrite structure. Similarly, large, single crystal alloys of Cu.sub.x Ag.sub.(1-x) InSe.sub.2 and CuIn.sub.y Ga.sub.(1-y) Se.sub.2 having chalcopyrite structures are not known to exist in the prior art.